1. Field of the Invention
The present invention relates to a current-voltage transforming (converting and amplifying) circuit employing a limiter circuit unit used with a photo detector integrated circuit, and more particularly, to a photo detector integrated circuit having a current-voltage transforming (converting and amplifying) circuit employing a limiter circuit unit detecting a current flowing through the current-voltage transforming circuit to generate a limiter current to the current-voltage transforming circuit, thereby preventing saturation of the current-voltage transforming circuit occurring when a high optical power is supplied to the photo detector integrated circuit during a writing operation of writing data on a recording medium, such as a CD-RW, in a disc recording and/or reading apparatus.
2. Description of the Related Art
Generally, an. optical pickup apparatus recording information on and/or reading information from a recording medium, such as a CD-RW, includes a photo detector integrated circuit (PDIC) detecting an optical beam reflected from an optical recording medium, such as an optical disc, containing information by a laser beam generated from a laser diode.
In a recordable CD, such as the CD-RW, a writing operation is performed by projecting the laser beam having a high power on a surface of the CD to form a pit on the surface of the CD. When an excessive power of the laser beam is supplied to the PDIC during the writing operation, a great amount of current generates to cause a current-voltage transforming (converting and/or amplifying) circuit of the PDIC to be saturated, thereby distorting an over-response characteristic of the current-voltage transforming circuit of the PDIC.
FIG. 1 is a view showing a conventional current-voltage transforming (converting and/or amplifying) circuit having a limiter circuit in a photo detector integrated circuit.
A photo detector (PD) current IPD is generated by carriers generated from a photo detector (PD) 12 to which an input laser beam is reflected. The PD current IPD pulls a feedback (FD) current If from an output buffer G3, the FD current If generates a voltage difference on both ends of the feedback resistor when flowing through the feedback resistor having a resistance of Rf, and the PD current IPD is converted into a voltage signal VOUT of VOUT=If×Rf.
A differential amplifier G1, an emitter follower G2, and the output buffer G3 constitutes a trans-impedance amplifier 10. If an excessive optical signal is inputted into the PD 12, the trans-impedance amplifier 10 is saturated to be out of an operational range. Due to this saturation of the trans-impedance amplifier 10, when the FD current If increases, the output voltage VOUT is no longer increased, an amplification speed of the current-voltage transforming circuit becomes slow even in response to a small amount of an input signal, and an excessive response characteristic of the current-voltage transforming circuit is distorted.
Referring to FIG. 1, an emitter of a transistor Q1 is connected to an output terminal of the emitter follower G2 to detect an output of the emitter follower G2. When an output voltage of the emitter follower G2 is greater than a predetermined value of VLMT+VBE, the transistor Q is turned on so that the limiter current ILMT is allowed to flow through the current-voltage transforming circuit. At a terminal Tin, IPD=If+ILMT. Even if the PD current IPD is excessively increased, the FD current If is no longer increased, and the trans-impedance amplifier 10 is not saturated but continues to operate.
As described above, in order to prevent the current-voltage transforming circuit from being saturated, an output voltage of the trans-impedance amplifier 10 is detected. When the output voltage is greater than a predetermined voltage, the limiter current ILMT is allowed to flow through the trans-impedance amplifier 10 so as to prevent the trans-impedance amplifier from being saturated.
In Japanese patent publication no.: 10-188315, disclosed on Jul. 21, 1998, an output voltage of a current-voltage transforming circuit is detected to allow a limiter current to flow into the current-voltage transforming circuit when the output voltage is greater than a predetermined value, so that saturation of the current-voltage transforming circuit is prevented.
The current-voltage transforming circuit disclosed in the above Japanese publication includes a first amplifier, a second amplifier, and an output buffer. An emitter of a transistor is connected to receive an output of the second amplifier, and the transistor is turned on to generate the limiter current ILT when the output of the second amplifier is greater than VREF+VBE, that is, V0>VREF+VBE. Therefore, a feed back current If is no longer increased even if a current IPH flowing through a photo detector is increased, so that the saturation of the current-voltage transforming circuit is prevented.
The above conventional current-voltage transforming circuit and the Japanese patent publication disclose that a current is compensated by detecting a voltage, and the Japanese patent publication discloses that the emitter of the transistor is connected to the second amplifier to perform a switching operation on the limiter current ILT as well as to control the output voltage V0 to be below VREF+VBE.
However, the circuit having a PNP type transistor is designed to limit a maximum value of a predetermined terminal voltage controlled according to a characteristic of the PNP type transistor, and the circuit having an NPN type transistor is designed to limit a minimum value of the predetermined terminal voltage controlled according to the characteristic of the NPN transistor.
In the above patent publication, since the maximum value of the output voltage V0 is controlled to below VREF+VBE, that is, the maximum value of the output voltage V0 is limited below VREF+VBE, circuit elements should not be the NPN type transistor but the PNP type transistor. Accordingly, a current-voltage transforming circuit disclosed in the above patent publication should include the PNP type transistor, which has a lower gain than the NPN type transistor. Therefore, it is disadvantageous that the gain is lowered, and a speed becomes slow.
In order to solve the above problems, it is needed to develop an improved current-voltage transforming (converting and/or amplifying) circuit which does not limit the output voltage of the amplifier, which constitutes the current-voltage transforming circuit, to a predetermined value regardless of the NPN type transistor and the PNP type transistor, and dose not lower the gain and the speed.